A mother in Washington noticed something unusual one day, just before her daughter's eighth birthday.

MOTHER: "I noticed around the end of two thousand eight, I think she's getting breasts. This is ridiculous, she’s seven, and I thought I was crazy. So, when we went for our annual check-up, I asked the doctor if I was right. And she said, 'Yeah, she has breast buds.'"

Last year a study in Denmark reported an increase in early breast development. Now a new study in the journal Pediatrics adds to evidence of an increase in early puberty in American girls.

The study took place in three big cities. Dr. Frank Biro at Cincinnati Children's Hospital Medical Center in Ohio led the study. He says it began about seven years ago.

FRANK BIRO: "We followed this group of approximately twelve hundred girls on a regular basis to see when they would enter into puberty."

Dr. Biro says the study found that white girls had the greatest increase in rates of breast development. The rates, he says, were about twice as high for seven- and eight-year-olds compared to earlier studies.

At age seven, the rates were ten percent of whites, twenty-three percent of blacks and fifteen percent of Hispanics.

Dr. Biro says girls with a higher body mass index are more likely to enter puberty early. Body mass index, or BMI, is a measure of weight in relation to height. But the nation's increase in overweight children may be only part of the explanation.

The researchers have been collecting blood and urine samples from the girls for testing. Dr. Biro says the tests are to look for chemicals from the girls' environment that could affect growth.

FRANK BIRO: "That is, chemicals that might mimic natural hormones that our body makes, or somehow interfere with how those hormones act on the body."

He says these chemicals could include growth hormones fed to farm animals, but they could also include chemicals such as phthalates. These are used to makes some kinds of plastics and other products.

FRANK BIRO: "In addition to the types and amount of food we eat, the other important factors are some of these chemicals. For example, personal care factors, such as cosmetics, such as suntan lotions, contain certain chemicals that could impact timing of maturation or other body functions."

But it is too early to know what the tests will show.

FRANK BIRO: "We'll need to wait for all the girls to enter puberty before we have a good grasp on what the contribution is of those environmental chemicals."

He says less research has been done to look for early puberty in boys. But he says some studies have shown that a higher BMI does not speed up development in boys, and may even slow it.

And that's the VOA Special English Health Report, written by Caty Weaver. I'm Pat Bodnar.